In a typical semiconductor wafer bonding process, such as wafer level bonding using low temperature fusion bonding, wafers are placed in a chamber to undergo surface activation prior to bonding. The surface activation process introduces plasma over a surface of the wafer to be bonded. Depending on the type of plasma used, various chemical properties of the activated surface may be altered to improve adhesion. Subsequently, the activated wafer surface is cleaned of contaminants and bonded (e.g., to another wafer) using a suitable process (e.g., low temperature fusion bonding). In wafer-level packaging (i.e., when two wafers are bonded face-to-face), both wafers may undergo surface activation prior to bonding.
Current surface activation processes typically use a closed-type process. In a closed-type surface activation process, gas is passed between a top and bottom electrode in a sealed chamber. The electrodes excite and ionize the gas molecules, forming plasma. An issue with this approach is particle contamination caused by the chemical interaction between the plasma and materials on the wafer surface. Particle contamination is particularly prevalent in hybrid wafers (i.e., wafers having more than two types of surface material) and especially in wafers having metal on its surface. Because close-type surface activation processes occur in sealed chambers, the particles are trapped in the sealed chamber and may damage the wafer or other wafers subsequently placed in the chamber.
Open-type surface activation processes involve exhausting the gas/plasma during the activation process and may reduce the presence of particle contamination. However, open-type surface activation reduces the density of plasma in the chamber, lowering the effectiveness of the activation process and weakening subsequently formed bonds.